Ординатура / Офтальмология / Английские материалы / LASIK and Beyond LASIK Wavefront Analysis and Customized Ablation_Boyd_2001
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Chapter 39
Figure 39-6. Example of image formed at the CCD plane using the optical diagram of figure X.
Figure 39-7. Typical output of wavefront measurement device. This is the eye of one of the authors (Luis Carvalho), which was done in October 2000 at the American Academy of Ophthalmology at the ZeissHumprhey booth. Notice the different plots: (upper left) HS image; (upper right) eye image; (lower left) color coded map of total aberrations; (lower right) color coded map of high order aberrations.
In figure 39-6 we may see an example of the type of images that are obtained with the optical setup in figure 39-5. The center of “mass” of each spot is detected using image-processing algorithms [28]. The and coordinates of each spot are then compared with coordinates of corresponding spots of a calibration image. The calibration image is obtained from an aberration free eye (emetropic), or more often from an artificial eye.
In figure 39-7 we present an eye wave-front measured with the HS sensor.
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416 SECTION V
WAVEFRONT MEASUREMENTS OF THE HUMAN EYE WITH HARTMANN-SHACK SENSOR
Present Technologies for Optimizing
Visual Acuity through Refractive
Surgery
In this section we’ll describe the current state of the art technologies for refractive surgery that are being tested and that will probably be available in the very near future.
Corneal Topography and Elevation
Maps
In the 1980’s, computer algorithms could do all measuring processes. The photographic camera was substituted by CCD cameras (from the words “ Charge Coupling Device”), and cards called “frame grabbers”, which grab images from the CCD into the computer memory. The manual and tiresome Placido image measurements could now be accomplished by image processing algorithms[28] and so the whole process would take only a few minutes. As computers grew more powerful, this process became faster and faster and with the popularity of colored monitors, the first high resolution corneal topography maps were plotted, originally suggested by Klyce[32]. These instruments are quite popular nowadays and became generally known as VKS (Videokeratoscopes) or Corneal Topographers.
Since the beginning of the computerized VKS, many authors have proposed different mathematical algorithms to calculate corneal features. It is important to notice that there are lots of parameters that may be calculated and that each of them have a different meaning. To make our point clear let’s look at the diagrams in figure 39-8:
Flying Spot Lasers and Eye Tracking
Recent excimer lasers, like the Nidek EC-5000[36], have a scanning slit delivery system that can treat over 7.5 mm of the cornea in myopia and up to 10 mm in hyperopic, using a 10 to 40 Hz frequency. A larger area ablation can be combined with a small area (1.0 mm) over a 10 mm diameter of the cornea.
Figura 39-8. Different descriptors for corneal surface, each one |
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with it’s own advantages and disadvantages for corneal diag- |
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nosis. The curvature maps (A and B) and the refractive map (C) |
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are particularly useful for pre and post-surgical evaluation, be- |
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cause they are proportional to corneal power. Descriptors D, E |
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and F are all elevation maps. They measure the “true” topogra- |
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phy of the cornea, relative to a plain (D), to a small sphere (E) |
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and to a big sphere (F). A detailed analysis of advantages and |
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disadvantages of each descriptor may be found in the works of |
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Salmon[33] and Klein[34]. |
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Authors like Ronald Krueger (Summit-Au- |
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tonomous Custom Cornea) argue that effective wave- |
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front based customized ablations require small scan- |
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ning spot gaussian beams[37]. In essence, Krueger |
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states that small gaussian beams allow for very |
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smooth ablation profiles, which directly affect post |
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surgical visual acuity. The other aspect to consider |
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is the size of the spot. If we have high-resolution |
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corneal topographers and wave-front devices, the size |
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of the laser beam has to be proportional to that reso- |
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lution. Unpublished mathematical calculations[37] |
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show that to correct up to fourth order aberrations a |
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spot size of less than 1 mm is necessary and there- |
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fore lasers with greater beam profiles would fail to |
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LASIK AND BEYOND LASIK 417
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correct common high order aberrations, like coma and spherical aberration. Studies with a 2 mm profile beam have shown poor performance[38].
Because of the involuntary constant movements of the eye (called saccadic movements) there is a need to correct eye position in order to place beam with precision. There are basically two types of eye tracking systems in the market: the CCD based systems and radar systems. The CCD based systems work with image processing algorithms to find eye position and input feedback to mirror mi- cro-motors; they have tracking frequencies that are limited by the CCD frequency and range from 30 to 300 Hz. Radar based systems work with retro-re- flected diode laser light and may obtain even higher frequencies[37].
A Look into the Future of Refractive Surgery
In this section we’ll make brief comments with references about certain aspects that, in our point of view, have yet to be considered in order to maximize refractive surgery efficiency.
Physiological Limitations to Visual Acuity
Although most people in the refractive surgery community show excitement with supernormal vision possibilities, we must consider the physiological limitations of the eye. No matter how well we can measure and correct low and high order aberrations, there is a clear limitation imposed by the human photoreceptor configuration and dimensions. The region of the retina where images are formed is the foveola, an approximately 0.35mm disk. In the foveola the cones are very packed and have a mean diameter of 2 m. Just like the number and size of photo sensitive cells in a CCD camera imposes limitation to the camera’s resolution, so does the number and size of cones in our eyes. Seeing with higher acuity essentially means seeing more detail at longer distances. It is easy to understand by simple image
formation principles from geometric optics, that smaller objects in the outer world will form smaller images at the retina. The question is: how small can an image formed at the retina still be interpreted? If it gets too small it will unavoidably be interpreted as a single point. A simple calculation may be done to show that visual acuity is limited in the retina to about 20/08. But there are certainly many other aspects that have to be considered in an individual basis, such as receptor sampling[46].
Considering Cyclotocion
Another interesting question that we think worthwhile is the consideration of cyclotorcion factors in refractive surgery procedures. Most pre and post surgery eye examinations are done in the vertical position of the head. But surgery takes place in the horizontal position. Our question is: how important is the cyclotorcion movements of the eye and head misalignment in cases of refractive surgery for cases of medium to high astigmatisms?
To answer this question we’ll make some theoretical calculations. Suppose a patient with 4 degrees of astigmatism with the rule (40D (8.43 mm of radius) at the horizontal meridian and 44D (7.67mm of radius) at the vertical meridian) undergoes refractive surgery and an accumulated meridian angle error of 5 degrees is caused by cyclototion and more 5 degrees because of head misalignment.
Let’s suppose that the simple refractive procedure would be to ablate the cornea in such a way as to flatten the steeper meridian, that is, the vertical meridian. So we know that the correct radius of curvature at the vertical meridian would be 8.43 mm. Simple geometrical calculations show that, for a radial distance of 1.0 mm from the apex of the cornea (which means 2 mm central region of the cornea) up to 0.2D errors may occur. Although theoretical, these simple calculations show that the head alignment and cyclotorcion are important aspects in customized ablations, since the precision of all instruments involved are much higher than 0.2D.
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418 SECTION V
WAVEFRONT MEASUREMENTS OF THE HUMAN EYE WITH HARTMANN-SHACK SENSOR
Effectiveness of the Hartmann-Shack
Sensor
It is important to consider optical design optimizations when using HS sensors to measure optical aberrations of the eye. Because the HS sensor was originally conceived for application in astronomy
and aberrations in this field differ from those of human eyes, we believe some studies have to be made in this sense. We have developed simulations of HS patterns for real and artificial corneal topography data of eyes with astigmatism, keratocone, and uniform curvature corneas.
The basic principle of HS simulation using ray tracing may be seen in figure 39-9.
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Figure 39-9. Ray-tracing diagram for generating the HS image pattern. We start by sampling pixels at the CCD array (480x640) and back-word ray trace from CCD plane towards the cornea. V1, V2 and V3 represent vectors at each refraction stage. Rays refract at micro-lens array then at cornea and finally hits the retina. If it falls inside the fovea (a 10 m disk) it is said to be a “good ray”, otherwise it is a “bad ray”.
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Illustration of simulated HS patterns obtained for three interesting cases are shown in figure 39-10. In general we notice that for eyes with little corneal irregularities (“smooth” corneas) the spots have a quite well behaved distribution; on the
other hand for eyes with high astigmatism, keratocone or other severe corneal irregularities (such as post RK), there is a superposition of the HS spots.
Our HS patterns are in agreement with the corneal elevation data and for most cases of regular
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Figure 39-10. Examples of HS simulations for a regular (top) and astigmatic (bottom) corneas. (Top-Left) Hartmann Shack pattern simulation for regular cornea; notice uniform distribution of spots; (top-middle) semi-meridian cut of regular cornea elevation; notice that curve is smooth and there is no local irregularities; (bottom-left) HS pattern for astigmatic eye; notice that spots are closer where corneal curvature is more intense and are further away for less curved region; (bottom-middle) Blue curve represents flatter meridian and red curve represents meridian with higher curvature; (bottom-right) curvature map of astigmatic eye, showing the “hour glass” shape in agreement with HS pattern and meridian cuts.
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(“smooth”) corneas, small and medium astigmatisms, there was no spot overlap (see figure 39-11). But for cases of severe keratocone (simulated), we observed overlapping (see figure 39-11). Other types of irregularities should be investigated, such as post-cataract, post-RK, and post-Keratoplasty. We believe there will be overlapping for these types of irregular corneas.
On figure 39-11 we may see examples of HS patterns obtained for artificial corneas generated using ellipsoids and spheres of different sizes and pa-
rameters. It is important to notice how the HS pattern varies with small changes in parameters such as radius of curvature, entrance pupil, HS image plane distance, number and size of micro-lenses, CCD resolution and scaling, and so on. Our objective here is to show a qualitative view of how these parameters affect the HS patterns. Further work should be done in order to quantify these factors, and possibly suggest HS sensor setups that will generate less superposition in cases of highly distorted corneas.
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Figure 39-11. HS patterns generated for simulated corneas. (a) Sphere of radius 8.0 mm, (c) Discentered Keratocone (to the left) with 5 mm local radius over a highly astigmatic ellipsoid (a:=7 mm, b:=5 mm, c:=8 mm), showing the superposition (to the left) case when the surface is off axis; (c) Highly astigmatic ellipsoid (a:=8 mm,
b:=5 mm, c:=7.5 mm), showing high distortion of HS patterns.
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31. Le Grand Y, El Hage SG, Physiological Optics, Springer |
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35.Hermann Von Helmholtz, Ed. by James P.C. Southall, Helmholtz’s Treatise on Physiological Optics, July 2000.
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37.Krueger R, Technology requirements for Summit-Autono- mous CustomCornea, Journal of Refract. Surg., 2000,Vol 16, 5: 592-601.
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tectomy ablations for the correction of spherical and cylindrical refractive error and higher-order aberration, J. Opt. Soc. Am. A/ Vol. 15, No. 9, September 1998, 2572-2579.
42.Tyson RK, Principles of adaptive optics, Academic Press, 1998.
43.Bille JF, Preoperative simulations of outcomes using adaptive optics, Journal of Refract. Surg., 2000,Vol 16, 5: 602607.
44.Williams D, Yoon GY, Porter J, Guirao A, Visual benefits of correcting higher order aberrations of the eye, Journal of Refract. Surg., 2000,Vol 16, 5: 554-559.
45.Roberts C, Future challenges to aberration free ablative procedures, Journal of Refract. Surg., 2000,Vol 16, 5: 623-629.
46.Applegate RA, Limits to vision: can we do better than nature?, Journal of Refract. Surg., 2000,Vol 16, 5: 547-551.
47.Carvalho LAV, Castro JC, Schor P, Chamon W, A software simmulation of Hartmann-Schack patterns for real corneas, International Symposium: Adaptive Optics: from telescopes to the human eye, Murcia, Spain, November 13-14, 2000.
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The authors have no financial interest in the products or brands presented in this chapter.
Information about the authors:
Luis Alberto Carvalho, PhD graduated in Physics from the University of São Paulo – Brazil where he also received his PhD. He also conducted research as a visiting scholar at the University of California – Berkeley - USA.
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Jarbas Caiado Castro, PhD, graduated in Physics from the University of São Paulo – Brazil, did his PhD at MIT-USA and is full professor at the Institute of Physcis of the University of São Paulo - Brazil.
Wallace Chamon, PhD, MD, graduated in Ophthalmology from the University of São Paulo – Brazil, conducted his PhD at the Escola Paulista de Medicina, and today is responsible for the Refractive Surgery Devision at that school. He has been fellowship and visiting scientist at the The Johns Hopkins University – USA and is Associate Editor of the Journal of Refractive Surgery.
Paulo Schor, PhD, MD, graduated in Ophthalmology from the University of São Paulo – Brazil, conducted his PhD at the Escola Paulista de Medicina, and today is responsible for Bioengineering Division of that school. He has been fellowship and visiting scientist at MIT and Harvard - USA.
Luiz Antonio Vieira de Carvalho, PhD, graduated in mathematics from University Júlio Mesquita Filho – Brazil, conducted his PhD at Brown University – USA in applied mathematics, and is full professor at University of São Paulo – Brazil.
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PRESBYOPIA
Chapter 40
PRESBYOPIA
Surgical Correction - Current Trends
Prof. Benjamin F. Boyd, M.D., FACS
With the Collaboration of:
Ronald Krueger, M.D. |
Prof. Juan Murube, M.D. |
Marguerite McDonald, M.D. |
Steven Wilson, M.D. |
Surgery for Management of Presbyopia through MONOVISION
Patients with a combination of mild hyperopia as well as presbyopia generally do not have a good visual adjustment unless they wear spectacles or contact lenses all the time. Before treating patients over 40, Ronald Krueger, M.D. discusses with them the concept of monovision. Monovision involves identifying the dominant eye and treating it in such a way that the patient can see as sharply as possible at distance. Then the non-dominant eye is treated but left a little myopic so the patient can still read, see at intermediate distance and at close range. Krueger estimates that 80-90% of his patients over the age of 40 report satisfaction with monovision surgery.
The LADARVision Laser for Myopia and Presbyopia (Monovision Method)
With a patient who is nearsighted, Krueger performs LASIK and uses the Autonomous LADARVision laser, by Alcon. For presbyopic patients, he corrects one eye for distance and leaves
the other eye slightly undercorrected, depending upon the patient’s age. For instance, Krueger may treat a patient in his early 40's to a target of -1.0D in the non dominant eye. At -1.0D, they should see everything at 1 meter away and maybe somewhat closer because they still have some accommodation.
He often targets patients in their late 40’s or early 50’s so they end up with -1.25D, and patients in their mid-50’s or older at about -1.5D. This allows patients to have maximum flexibility with mid-range and near vision in one eye. The other eye, as long as it is very sharply focused at distance, will give them the clear vision without glasses that they want.
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Hyperopia and Presbyopia
(Monovision Method)
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In patients with hyperopia, Krueger treats the dominant eye for the hyperopia prescription so they can see well at distance. Then he overcorrects the non-dominant eye to bring patients from hyperopia, beyond emmetropia to mild myopia. This is still a monovision treatment. Often patients require slightly less myopia in the non-dominant eye as the hyperopic correction gives them a certain amount of negative asphericity (steeper in center) which favors better near vision when the pupil constricts.
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